MÖSSBAUER STUDY ON RECOVERY OF COLD-WORKED Fe-Al ALLOYS

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MÖSSBAUER STUDY ON RECOVERY OF COLD-WORKED Fe-Al ALLOYS

K. Oki, S. Towata, M. Tamiya, T. Eguchi

To cite this version:

K. Oki, S. Towata, M. Tamiya, T. Eguchi. MÖSSBAUER STUDY ON RECOVERY OF COLD- WORKED Fe-Al ALLOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-611-C2-612.

�10.1051/jphyscol:19792212�. �jpa-00218592�

JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3 , Tome 40, mars 1979, page c2-611

MOSSBAUER STUDY ON

OF

ALLOYS

K. Oki, S. Towata, M. Tamiya and T. Eguchi

Department o f Materials Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812, J a p m

Rbsum6.- Le processus de la restauration isotherme de l'ordre atomique dans les alliages Fe-A1 dcrouis a Btd dtudid par spectrometric Mb'ssbauer. On a analysQ les spectres observes pour obtenir 2i chaque Btape du recuit la distribution du champ interne dans les alliages. La probabilit6 d'existence des configurations atomiques a pu Stre obtenue aussi, en supposant une fonction de distribution Gaussiem du champ hyperfin. Les alliages ordonnds de type DO,, magnetiques ou non-magnetiques selon leur compo- sition, ont 6t6 ddsordonn6s et rendus magn6tiques par laminage. Pour ces alliages, on assiste 2i une restauration trPs lente,au cours de recuit, vers l'gtat initial de type DO, en passant par un ordre de type B,. Les alliages ordonnds de type B,, non-magndtiques, ont 6td partiellement ddsordonnds par laminage et ont donnd un spectre Mb'ssbauer comportant des caractsristiques magndtiques et non-magndti- ques. Les alliages laminds se sont rapidement rQordonn6s pour donner les Qtats initiaux par le recuit.

Abstract.- The process of atomic reordering of cold-worked Fe-A1 alloys on isothermal annealing has been investigated by means of ~b'ssbauer spectroscopy. Observed spectra were analyzed to obtain the distribution function of the internal magnetic field in the alloys at each stage of annealing. The probabilities of the nearest neighbor configurations of constituent atoms could also be obtained by assuming the Gaussian distribution function of hyperfine field acting on iron atoms with various num- ber of iron neighbors. The alloys ordered with DO3 symmetry, which are either magnetic or nonmagnetic depending on their composition, were changed by cold working into disordered and strongly magnetic states. These alloys recovered very slowly in the course of annealing to their initial state by way of B, type of order. The alloys with B2 type of order, which are nonmagnetic, were partially disorder- ed by cold working, and exhibited the Mb'ssbauer spectra of both magnetic and nonmagnetic characters.

The alloys recovered promptly to their initial state by annealing.

1. Introduction.- It is known that cold working of ingots were then homogenized at 900°C for 24h., made ordered Fe-A1 alloys leads to some remarkable changes order by slow cooling, and filed into powder by a in their crystallographic and magnetic properties

11-4/. The ordered alloy with B, type is nonmagnetic, while the crushed alloy is weakly magnetic. The phe- nomena are explained by considering the nearest neighbor configurations of iron atoms /2,3/. In the present work the process of atomic reordering of cold-worked Fe-A1 alloys was investigated by using a Mijssbauer spectroscope, and a quantitative analysis was attempted of the data obtained from the deformed sample. Massbauer spectra from the specimens filed and annealed were observed and analyzed to obtain the distribution function of the internal magnetic field. Furthermore, another approach was made to ac- quire information on the nearest neighbor configura- tions of constituent atoms directly from the

MSSS-

bauer spectra under an appropriate assumption as to the hyperfine field acting on i-nn Fe (an iron atom with i nearest iron neighbors). This method enables us to trace the variation of atomic configurations accompanied by the recovery of ordering in plasti- cally deformed materials.

2. Experimental.- The Fe-A1 alloys with 25 to 35 at.

% A1 were prepared by vacuum induction melting from 99.95% electrolytic iron and 99.99% aluminium. The

diamond instrument. The powders were sealed in eva- cuated quartz tubes, annealed isothermally for var- ious lengths of time, and then quenched into iced brine. The Mb'ssbauer experiments were carried out at room temperature using a Shimadzu YEG-IA type spec- trometer, and the absorption spectra of the specimens filed and annealed were observed with a 10mCi source of 5 7 ~ o in palladium.

3. Results.- Figure 1 shows the typical Mb'ssbauer spectra of 25 at.% A1 as-filed or filed and annealed at 400'~ and the corresponding P(H) curves (the dis- tributions of internal field) obtained by following the method of Hesse et al. 151. The alloy forms the superstructure with DO3 type at room temperature and exhibits two peaks in the P(H) curve at about 210 and 300 kOe corresponding to the internal fields acting on the 4 and 8 nn F e , respectively. As-filed alloy has a spectrum with a smooth P(H) curve, which indicates that the atomic ordering of DO, type has been destroyed by cold working alntost into the dis- ordered state. With the progress of annealing the peaks for the 4 and 8 nn Fe become higher, and it is apparent that the recovery to the DO3 state is pro- ceeding. The alloy with 31.5 at.% Al, which is in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792212

C2-612 JOURNAL DE PHYSIQUE

the DO3 type of order at room temperature exhibiting Figure 2 shows a set of probabilities for various nn the nonmagnetic single line spectrum, changes by cold Fe obtained from the Mijssbauer spectra shown in fig- working to the magnetic state. The filed alloy recov- urel. Here the distribution function of the Gaussian

ered very slowly to the DO, state by annealing. type was adopted as to the hyperfine field on each nn Fe. Though the internal fields of the alloy depend

Fig. 1 : ~zssbauer spectra of 25 at.% A1 alloy and the corresponding distributions of internal field : (a) and (a') as-filed; (b) and (b') annealed at 400°C for 3 min.; (c) and (c') 1000 min.; (d) and

(d') slowly cooled.

The 35 at.% A1 alloy, which is in the B 2 type of order and nonmagnetic at room temperature, gives on cold working a spectrum showing a nonmagnetic compo- nent and a magnetic one with large internal fields.

The former component is due to the presence of some of the B2 phase still remaining after filing. The alloy recovered promptly to its initial state even in a low temperature annealing.

primary on the number of iron atoms on the first nearest shell / b / , the effect of second and farther nearest neighbors was taken into account as the width of the Gaussian function.

Fe 25al%AI 4OO.C Anneal

j

I

I -I

Fig. 2 : Change of probabilities of various nn Fe for 25 at.% A1 alloy on isothermal annealing at 400'~.

As is seen in the figure the probabilities for 4 and 8 nn Fe increase while those for 5 and 7 nn Fe de- crease with annealing time. The variation of these probabilities demonstrates the progress of DO type reordering. The probability of 4 nn Fe increases after the saturation of the one for 8 nn Fe. This tendency is more remarkable when the alloy is anneal- ed at 250°C, and implies that this alloy recovered to DO, by way of B, state.

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